Traditional polyurethane containing coating compositions employ large quantities of organic solvents. Because they are able to undergo chemical crosslinking at low temperatures and harden fast, and coatings prepared from them possess very good chemical and physical properties, they have long been widely employed in the surface treatment of articles such as fabrics, leathers, flexible PVC sheets, and the like. In recent years, due to the escalated requirements in environmental protection, industrial safety, and employee health, coating resins employing large quantities of organic solvents can no longer satisfy the needs and have gradually become obsolete. Generally accepted substitutes are water dispersible polyurethanes. However, up to date, water dispersible polyurethanes still have many problems in their production methods and the films formed by the latices containing such polymers have not been found to be entirely satisfactory with respect to sufficient hardening, curing or crosslinking under ambient conditions, chemical and physical properties, which prevent them from being widely used.
Many methods of preparing water dispersible polyurethanes are known in the art. They can be generally classified as surfactant-containing and surfactant-free methods. Due to the fact that residual surfactants can hurt coating properties, surfactant-containing methods are seldom used. Surfactant-free methods are more widely employed. A typical surfactant-free method is to first prepare NCO-terminated urethane prepolymer, and then let the prepolymer undergo salt-forming and chain-extending processes and eventually disperse into water. The production processes of surfactant-free methods can be further categorized into organic solvent-containing and organic solvent-free methods.
The organic solvent-containing methods produce resins with a higher molecular weight and better properties. However, after the preparation of aqueous dispersion, organic solvents have to be removed. As a result, the process is of low productivity and requires a long process time. Furthermore, it is often complicated by defoaming difficulties and other problems that have strong effects on productivity and manufacturing costs. There are methods that employ approximately 10-20% by weight of low odor, hydrophilic organic solvents such as N-methyl pyrolidone, NMP, and require no removal after being dispersed into water. However, NMP's low volatility and high hydrophilicity greatly reduce the drying rate of coating films (especially at room temperature or low temperature). Thus, it is suitable for use only in high temperature baking processes. Nevertheless, the use of organic solvent is not favored in environmental consideration. Examples of organic solvent-containing process can be found in U.S. Pat. Nos. 4,046,729, column 14; 4,066,591, column 14; 4,147,679, column 14; 4,203,883, columns 6-7; and 4,335,029, columns 4-5.
There are organic solvent-free methods, such as Melt Dispersion Process, Prepolymer Mixing Process and Ketimine Process. These methods have common limitations of having to operate in at high temperature and high viscosity environments, and have gelation and other unsurmountable difficulties in actual manufacturing. As a result, these methods are seldom employed in industrial manufacturing. Furthermore, polyisocyanates suitable for these methods are limited to aliphatic diisocyanates, which are of low water reactivity and extremely expensive. Diisocyanates of low cost but higher water reactivity such as TDI are not suitable for the methods. As a result, the raw material costs of these methods are in great disadvantage.
Comparing water dispersible polyurethanes prepared by the methods described above with those prepared by traditional, organic solvent-based methods, the former show inferior physical and chemical properties due to their difficulties in undergoing chemical crosslinking reactions. Furthermore, the hydrophilicity of the residual salts also results in inferior properties. If polyurethanes are crosslinked before being dispersed into water, their low meltability and low solubility would result in operational difficulties in manufacturing processes, and it would be difficult to disperse them into water. Even if they were dispersed into water, the storage stability of the aqueous dispersion would be poor. Thus, in order to improve its properties, common practices are to add crosslinking agents into the aqueous dispersion of polyurethanes. Up to now, the majority of crosslinking agents can only undergo crosslinking reaction at high temperatures. and their curing rate is not satisfactory to the high speed requirements common to industrial productions. Although there are crosslinking agents such as polyaziridines and polycarbodiimides that undergo crosslinking at low temperatures, their use is still limited by shortcomings such as low curing rate, low stability, toxicity and poor coating properties.